U.S. patent application number 12/482410 was filed with the patent office on 2009-12-17 for radio frequency identification system with improved accuracy and detection efficiency in presence of clutter.
Invention is credited to Ashis Khan, Somnath Mukherjee.
Application Number | 20090309706 12/482410 |
Document ID | / |
Family ID | 41414215 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090309706 |
Kind Code |
A1 |
Mukherjee; Somnath ; et
al. |
December 17, 2009 |
RADIO FREQUENCY IDENTIFICATION SYSTEM WITH IMPROVED ACCURACY AND
DETECTION EFFICIENCY IN PRESENCE OF CLUTTER
Abstract
A technique that improves performance of passive backscatter
RFID tags such as mitigation of read error in presence of clutter,
provide enhanced range, speed up anti-collision reading, provide
increased throughput etc. The technique utilizes amplitude and
phase modulation at the tag and a compensation algorithm at the
RFID reader without inflicting significant changes in the RFID chip
and therefore has minimum cost impact. Modifications can be
primarily in the antenna design and passive circuitry around it,
printable by a single step process.
Inventors: |
Mukherjee; Somnath;
(Milpitas, CA) ; Khan; Ashis; (San Jose,
CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Family ID: |
41414215 |
Appl. No.: |
12/482410 |
Filed: |
June 10, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61131765 |
Jun 11, 2008 |
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Current U.S.
Class: |
340/10.1 |
Current CPC
Class: |
H01Q 1/2225 20130101;
H01Q 1/2208 20130101; H04B 5/0062 20130101 |
Class at
Publication: |
340/10.1 |
International
Class: |
H04B 7/00 20060101
H04B007/00 |
Claims
1 A passive backscatter radio frequency identification (RFID) tag,
comprising: a non-dipole antenna; a switch connected to the
non-dipole antenna; and at least one circuit having a predetermined
impedance, the switch adapted to switch an electrical connection
between the non-dipole antenna and the at least one circuit such
that the RFID tag emits a predetermined pattern of signals when
illuminated by an RFID interrogator, the pattern of signals
corresponding to a device signature of the RFID tag.
2. The RFID tag of claim 1 wherein the antenna is of an antenna
type that emits radio frequency backscatter when the antenna is
terminated by a reactive circuit and the antenna is illuminated by
an interrogator.
3. The RFID tag of claim 2 wherein the reactive circuit is an open
circuit.
4. The RFID tag of claim 1 wherein the antenna has a main element
and at least one parasitic element.
5. The RFID tag of claim 1 wherein the antenna comprises a patch
antenna.
6. The RFID tag of claim 1 wherein the at least one circuit
comprises at least four circuits and the at least four circuits are
adapted to emit a quadrature amplitude modulation (QAM) signal from
the RFID tag.
7. A passive backscatter radio frequency identification (RFID) tag,
comprising: an antenna; an electrical switch; a first circuit
element having a first impedance; a second circuit element having a
second impedance; and the switch adapted to switch from an
electrical connection between the antenna and the first circuit
element to an electrical connection between the antenna and the
second circuit element when the RFID tag is illuminated by an RFID
interrogator such that the RFID tag emits a predetermined pattern
of signals when illuminated by an RFID interrogator, the pattern of
signals corresponding to a device signature of the RFID tag.
8. The RFID tag of claim 7 wherein the antenna is of an antenna
type that emits radio frequency backscatter when the antenna is
terminated by an open circuit and the antenna is illuminated by an
interrogator.
9. The RFID tag of claim 7 wherein the antenna has a main element
and at least one parasitic element.
10. The RFID tag of claim 7 wherein the antenna comprises a
non-dipole antenna.
11. The RFID tag of claim 7 wherein the antenna comprises a patch
antenna.
12. The RFID tag of claim 7 wherein the first and second circuit
elements have distinct reactances, the distinct reactances adapted
to modulate a phase of an illumination signal from an RFID
interrogator.
13. The RFID tag of claim 7 wherein the first and second circuit
elements have distinct resistances, the distinct resistances
adapted to modulate an amplitude of an illumination signal from an
RFID interrogator.
14. The RFID tag of claim 1 wherein the first and second circuit
elements have distinct resistances and distinct reactances, the
distinct resistances and reactances adapted to modulate both an
amplitude and a phase of an illumination signal from an RFID
interrogator.
15. A method of impedance modulation of a passive backscatter radio
frequency identification (RFID) tag, the method comprising:
receiving a wireless input signal into an antenna of an RFID tag,
the antenna operatively connected to a switch; switching the switch
between a first circuit having a first impedance and a second
circuit having a second impedance such that an output signal from
the antenna is modulated both in a predetermined amplitude and a
predetermined phase, the modulation in both the predetermined
amplitude and the predetermined phase corresponding to a device
signature of the RFID tag; and emitting the output signal from the
RFID tag.
16. The method of claim 15 wherein the first and second circuits
are both substantially lossless, the impedances of the lossless
circuits corresponding to the predetermined phase.
17. The method of claim 16 further comprising switching the switch
between a plurality of circuits, each having an element selected
from the group consisting of an open circuit, a short circuit, an
inductive circuit, and a capacitive circuit.
18. The method of claim 15 wherein the antenna has a main element
and at least one parasitic element.
19. The method of claim 18 wherein at least one of the first and
second circuits dissipates power, such that the dissipation
corresponds to the predetermined amplitude.
20. The method of claim 15 further comprising switching the switch
between the first and second circuits and a third circuit having a
third impedance and a fourth circuit having a fourth impedance.
21. The method of claim 20 further comprising generating a
quadrature amplitude modulation (QAM) signal.
22. A machine-implemented method of discerning a radio frequency
identification (RFID) tag among non-RFID tag clutter, the method
comprising: receiving a wireless signal into an antenna, the
wireless signal including an emitted signal from an RFID tag and
clutter; estimating an amplitude and phase of the signal in two or
more distinct states; estimating an amplitude and phase of the
clutter; and subtracting the clutter from the received signal.
23. The method of claim 22 wherein the estimating an amplitude and
phase of the signal in two or more distinct states comprises:
computing an estimate of a low state of the RFID tag; and computing
an estimate of a high state of the RFID tag.
24. The method of claim 23 further comprising: setting first phase
shift states at the tag; determining whether a phase difference
between the computed estimates of the low and high states of the
RFID tag is discernable; and setting second phase shift states at
the tag based on a determination that the phase difference between
the computed estimates of the low and high states of the RFID tag
is not discernable.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/131,765, filed Jun. 11, 2008, hereby
incorporated by reference in its entirety for all purposes.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present disclosure relates generally to the field of
electrical communications and radio wave antennas as well as
remotely monitoring identification devices using electromagnetic
waves. More particularly, it relates to the field of radio
frequency identification (RFID) devices, sometimes called tags, in
which a tag is interrogated by a probing platform, sometimes called
a reader. Even more particularly, it relates to the field of radio
frequency identification tags that allows phase modulation or a
variation thereof (e.g. phase shift keying) in addition to
amplitude modulation for the tag-to-reader communication. Even more
particularly, it relates the field of radio frequency
identification systems employing advanced techniques such that
undesired clutter from non-tag items can be mitigated using a
compensation technique.
[0004] 2. Description of the Related Art
[0005] Radio-frequency Identification Devices (RFID) are used in
multiple asset tracking applications, e.g., automotive, airline
baggage, consumer items, food items, garments, livestock etc. There
are numerous medical and military applications too. Many RFID tags
do not use a battery for power. These types of RFID tags
predominantly use the principle of "passive backscatter" while
employing a semiconductor chip that converts part of the received
radio-frequency (RF) to DC power to energize the chip itself, as
described in common literature such as K. Finkenzeller, RFID
Handbook: Fundamentals and Applications in Contactless Smart Cards
and Identification, 2.sup.nd ed. (San Francisco, Calif.: John Wiley
& Sons, 2003). The RFID ecosystem operates in multiple bands,
e.g., low frequency (LF), high frequency (HF), ultra high frequency
(UHF) and microwaves and usually employs amplitude shift keying
(ASK) (or variations thereof) on the backscatter from the tag to
carry the information embedded in the tag to the reader. LF and HF
systems commonly operate under near-field conditions and have a
limited range of 1 meter or so, whereas UHF and microwave tags
predominantly operate under far-field conditions and are usually
capable of longer range and higher data rates, as described in
Smail Tedjini et al., "Antennas for RFID Tags" (Grenoble, France:
Joint sOc-EUSAI Conference, October 2005). The modulation and
coding formats are defined by various standards such as EPCglobal
Class 0, Class 1, Gen II, etc. for UHF tags.
[0006] A passive backscatter tag does not transmit its own signal
to the reader but simply modulates the signal that its antenna
backscatters by changing the impedance presented to the antenna. In
this fashion, the tag need only provide a switching function
operating at a modest rate comparable to the data rate of a few
hundred kbps, as described in D. Dobkin et al, "A Radio-Oriented
Introduction to Radio Frequency Identification," High Frequency
Electronics, June 2005. A typical UHF RFID tag uses a dipole
antenna (or a variation thereof) and is switched between open
circuit and a matched load. In other words, the data from tag to
reader is sent by amplitude modulating the radar cross section
(RCS) during the time interval that the tag receives a continuous
wave (CW) signal from the reader.
[0007] The performance of the tag to reader link is limited due to
the amplitude modulated nature of the signal. Amplitude modulated
signals usually require higher signal to noise ratios than the
phase modulated counterparts like phase-shift keying (PSK). A PSK
system will therefore provide longer range and be more tolerant to
multipath fades. Moreover, a multiple level PSK system can be used
to speed up the anti-collision reading process by assigning unique
phase states to different categories of tags. Also, a phase
modulation based system ideally can scatter back all the energy
captured by the tag from the reader, minus the amount converted to
DC. Furthermore, it does not require a precise impedance matching
as commonly required in ASK systems.
[0008] ASK based systems suffer major limitations in the presence
of reflected and scattered clutter from non-tag items, as well as
leakage of reader transmissions into the reader receiver.
[0009] Using coherent PSK detection at the reader, and adding a
compensation scheme, it is possible to separate these impairments
from the desired backscatter from the tag. However, conventional
methods to create phase modulated backscatter increases the
complexity of the chip inside the tag and thereby increase
cost.
[0010] A major obstacle in successful deployment of RFID tags is
the lack of ability of detecting multiple RFID tags accurately by
an RFID reader in a cluster. For example, an RFID reader may be
asked to read 100 items in a supermarket cart with all of the items
having RFID tags. Existing RFID systems have poor accuracy in
accurately detecting all such IDs in parallel due to clutter.
[0011] Thus, a better solution is needed to extend the range of
RFID tags, make operation robust in multipath situations, mitigate
the effect of clutter and speed up reading among a cluster.
BRIEF SUMMARY OF THE INVENTION
[0012] Embodiments in accordance with the present disclosure relate
to remote identification devices that illuminate a radio frequency
identification (RFID) device (e.g., an RFID tag) with
electromagnetic waves and coherently process the backscatter, the
RFID devices according to various embodiments are capable of
introducing amplitude and or phase modulation on the backscattered
signal with an on/off (i.e. single pole single throw) switch, and
an interrogator is equipped with a correction algorithm to mitigate
the effect of clutter resulting from reflection/scattering from
items containing electrical conductors (metal) or high dielectric
constant such as water based liquids, as is common in a supermarket
cart. Additional benefits are longer range, enhanced read speed in
a cluster of devices (anti-collision reading) and enhanced
throughput.
[0013] Passive backscatter RFID tags can receive a continuous wave
(CW) signal from a reader or interrogator and convert a part of the
wave into direct current (DC) used to power a chip inside the tag.
The chip in the RFID tag decodes the information sent out by the
reader and prepares a response. The response is a bit stream
generated by impedance modulating the tag antenna.
[0014] One embodiment relates to a method of impedance modulation
of a passive backscatter radio frequency identification (RFID) tag.
The method includes receiving a wireless input signal into an
antenna of an RFID tag, the antenna having a characteristic
impedance and being operatively connected to a switch, switching
the switch between a first circuit having a first impedance and a
second circuit having a second impedance such that an output signal
from the antenna is modulated both in a predetermined amplitude and
a predetermined phase, the modulation in both the predetermined
amplitude and the predetermined phase corresponding to a device
signature of the RFID tag, and emitting the output signal from the
RFID tag.
[0015] Another embodiment relates to a machine-implemented method
of discerning a radio frequency identification (RFID) tag among
non-RFID tag clutter. The method includes receiving a wireless
signal into a reader antenna, the wireless signal including an
emitted signal from an RFID tag and complex clutter, estimating an
amplitude and phase of the signal in two or more distinct states,
estimating an amplitude and phase of the complex clutter, and
subtracting the complex clutter from the received signal.
[0016] Yet another embodiment relates to a passive backscatter
radio frequency identification (RFID) tag. The tag includes a patch
element, a switch connected to the patch element, and at least one
circuit, each of the at least one circuit having a distinct
predetermined impedance. The switch is adapted to switch an
electrical connection between the patch element and the at least
one circuit such that the RFID tag emits a predetermined pattern of
signals when illuminated by an RFID interrogator, the pattern of
signals corresponding to a device signature of the RFID tag.
[0017] The foregoing has outlined, in general, the physical aspects
of the invention and is to serve as an aid to better understanding
the more complete detailed description that is to follow. In
reference to such, there is to be a clear understanding that the
present invention is not limited to the method or detail of
construction, fabrication, material, or application of use
described and illustrated herein. Any other variation of
fabrication, use, or application should be considered apparent as
an alternative embodiment of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The following drawings further describe by illustration the
advantages and objects of the present invention.
[0019] FIG. 1 illustrates how a reflection coefficient of a passive
network at an antenna port can change the relative amplitude and
phase of a backscattered signal in accordance with an
embodiment.
[0020] FIG. 2 shows a two-port representation of a non minimum
scatter antenna in accordance with an embodiment.
[0021] FIG. 3 describes a basic principle of a tag in accordance
with an embodiment in which the tag antenna port is switched
between N (where N.gtoreq.2) passive networks.
[0022] FIG. 4 depicts a microstrip patch as a candidate for the tag
antenna possessing the requisite property (non minimum scatter)
described in FIG. 1.
[0023] FIG. 5 shows a schematic for a preferred embodiment for
generating binary phase shift keying (BPSK).
[0024] FIG. 6 shows a schematic for a preferred embodiment for
generating quadrature phase shift keying (QPSK).
[0025] FIG. 7 is a phasor diagram illustrating how differential
phase shift can be utilized to recover tag modulation in accordance
with an embodiment, even in presence of heavy impairments
(clutter).
[0026] FIG. 8 is a plot from mathematical modeling depicting a
differential phase shift as a function of clutter phase angle for
180.degree. and 90.degree. keying, in accordance with an
embodiment.
[0027] FIG. 9 depicts logic followed in an embodiment of the reader
to recover tag modulation in presence of heavy impairments
(clutter) in accordance with an embodiment.
[0028] FIG. 10 depicts an algorithm for mitigating impairments
(clutter) in accordance with an embodiment.
[0029] FIG. 11 illustrates an RFID reader and tag in accordance
with an embodiment.
[0030] The figures will now be used to illustrate different
embodiments in accordance with the invention. The figures are
specific examples of embodiments and should not be interpreted as
limiting embodiments, but rather exemplary forms and
procedures.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The present disclosure describes a novel technique and
apparatus for modulating a backscattered signal from a radio
frequency identification (RFID) device or tag. This technique does
not require changes to the chip architecture inside an RFID tag.
Generally, the only changes that are needed are within the tag
antenna and associated circuitry. Because the tag antenna and
associated circuitry can be fabricated using the same process as a
typical tag antenna (e.g. single step printing) there is almost no
additional cost to the tag.
[0032] This novel technique modulates the phase of a backscattered
signal in addition to the amplitude. In one embodiment, this is
implemented through connecting the antenna to relatively low cost
and simple circuits that are external to the antenna through a
switch. In another embodiment, low cost and simple circuits are
integrated within the antenna.
[0033] To achieve control over both amplitude and phase modulation
of the backscattered signal, a special category of antennas that
scatters back a negligible quantity of signal energy when
terminated by its characteristic impedance is used. These types of
antennas almost invariably include at least one parasitic element
in addition to the main element and may be termed "non-minimum
scatter antennas" (see Mukherjee, S., "Antennas for Chipless Tags
Based On Remote Measurement of Complex Impedance," Proceedings of
the 38th European Microwave Conference (Amsterdam, 2008)). The
residual scattered power, under matched condition is due to
structural scattering. By proper design, the structural scattering
can be minimized.
[0034] One type of non-minimum scatter antenna is a "non-dipole
antenna" because a dipole antenna is generally not suitable as a
non-minimum scatter antenna. A non-dipole antenna generally can
create a backscatter even when terminated by an external, lossless
network, such as an open circuit or a short circuit.
[0035] The magnitude and phase of the scattered signal can be
controlled by the mismatch between the antenna characteristic
impedance and a passive external termination. Therefore, by proper
selection of two (or more) passive networks connected through a
switch to the antenna, amplitude and phase modulation of the
backscatter is achieved.
[0036] If the external termination is (ideally) lossless, only the
phase of the scattered signal is modulated. In this special case, a
phase-shift keyed (PSK) system would be implemented. If the
external network also includes a dissipative or resistive part in
addition to a reactive part, both amplitude and phase modulations
(e.g. quadrature amplitude modulation (QAM)) are achieved.
[0037] A received signal at a reader undergoes signal processing
with an algorithm that uses the amplitude and phase information
from the tag to mitigate the effect of clutter, usually coming from
reflection scattering from metallic objects, water based liquids
and leakage from the reader's transmit antenna.
[0038] There are several advantages to this technique. A PSK signal
will operate with substantially lower received power than amplitude
shift keying (ASK) and therefore increase the range of operation of
the reader and tag. The system can be more frugal in utilizing the
captured radio frequency (RF) power at the tag. Except for the
power converted to direct current (DC), generally all power is
scattered back to the reader. This is unlike RFID dipole antennas
in which half the power is dissipated in the antenna during the
`mark` mode (no power scattered back during `space` at all).
Because the RFID device using PSK is more frugal with the power,
this can allow operation with less power transmitted from the
reader. This can reduce interference, help to comply with local
regulatory standards, etc. Critical impedance matching between the
tag antenna and the chip is not required since the antenna is
operating almost always in mismatched condition for a PSK
operation. Tags almost always operate in the presence of clutter
coming from reflection/scattering from non-tag items and leakage
from the transmit antenna of the reader. It is possible to mitigate
the effect of this clutter by use of a compensation scheme
described. It is possible to speed up an anti-collision read
mechanism by creating several categories of tags and each category
imbibed with unique phase states. Also, the tag to reader data rate
can be increased through the use of an m-QAM system
[0039] Though Phase Shift Keying (PSK) is mentioned in the
EPCglobal Gen II standard for Tag to Reader communication (see EPC
Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID
Protocol for Communications at 860 MHz-960 MHz, version 1.1.0
(EPCglobal Inc., Dec. 17, 2005), section 6.3.1.3.1 on p. 27),
usually various forms of Amplitude Shift Keying (ASK) are used due
to simplicity of implementation. Amplitude shift keying of the
backscatter is implemented by alternately terminating the tag
antenna with its characteristic impedance and opening it. In other
words, the radar cross section (RCS) of the tag undergoes amplitude
modulation.
[0040] Several issues are evident from this disclosure: [0041] 1.
It is possible to design certain class of antennas that ideally
scatter back the entire signal captured provided the termination is
purely reactive. An example is microstrip patch and a
counter-example is a dipole. [0042] 2. The above category of
antennas scatters back a negligible quantity of signals when
terminated by the antennas' characteristic impedances. The residual
scattered power under such conditions can be called structural
scattering. By proper design, the structural scattering can be
minimized. This type of antenna can be termed a non minimum scatter
antenna. [0043] 3. The magnitude of the scattered signal can be
controlled by the mismatch between the antenna characteristic
impedance and the external termination. [0044] 4. The phase of the
scattered signal can be controlled by the reactive part of the
external network. [0045] 5. By the use of a switch to switch
between two or more external networks, it is possible to control
amplitude and phase of backscattered signals for multiple states.
It is therefore not necessary to construct a separate
amplitude/phase modulator in the tag chip.
[0046] FIG. 1 shows a non-minimum scatter antenna 102 terminated by
a passive network 103. Non-minimum scatter antenna 102 is
preferably an antenna on an RFID tag. Table 1 below depicts
relative amplitude and phase values for the scattered signal for
some typical passive networks. For example, a (perfectly) matched
termination to antenna 102 results in zero backscatter. For a
resistive and reactive network with a reflection coefficient in the
Euler complex phasor notation of Ae.sup.j.alpha., (where j is the
square root of negative 1), the relative amplitude of backscatter
is A (where A<1) and the relative phase of the backscatter is
.alpha..
TABLE-US-00001 TABLE 1 Amplitude of Phase of Backscatter
Backscatter Type of Passive Network (relative) (relative) Matched
Termination 0 n/a Open Circuit 0 0 Short Circuit 1 0 Lossless
network of 1 .phi. reflection coefficient 1 e.sup.j.phi. Lossy
network of A .alpha. reflection coefficient A e.sup.j.alpha.
[0047] FIG. 2 illustrates passive network 103 of FIG. 1 as passive
network 201. Passive network 201 is represented by a reflection
coefficient .GAMMA., and the antenna 102 replaced by its equivalent
two-port network 203. The source impedance Z0 204 is 120.pi. ohms,
i.e. the impedance of free space. The variable a is a transmitted
wave, and the variable b is a reflected wave. Variables .gamma.11
and .gamma.12 are propagation functions (i.e. a measure of
attenuation and phase shift due to propagation. Variables s11, s12,
s21, and s22 are scattering parameters. For an ideal non minimum
scatter antenna, s11 and s22 are both zero, and (normalized)
s21.times.s12=1.
[0048] FIG. 3 depicts passive backscatter RFID tag 300 in
accordance with an embodiment. Tag 300 has a non minimum scatter
antenna 302, which has a characteristic impedance, and an N
position (i.e. N-tuple throw) switch 306. N distinct
amplitude-phase states can be created by connecting N passive
networks 303, 304, . . . 305 to antenna 302 through switch 306.
Each passive network operatively connected to the i'th switch
position is represented by A.sub.ie.sup.j.phi.i, which represents a
predetermined reflection coefficient resulting from a predetermined
impedance.
[0049] In the exemplary embodiment, a wireless signal (not shown)
is received into antenna 302. Because switch 306 is connected to
passive network 303, incident wave 307 travels from antenna 302 to
passive network 303. Incident wave 307 is modified by passive
network 303 to create a reflected wave 309. Reflected wave 309
travels back through switch 306 and is radiated by antenna 302.
[0050] Passive network 303 modifies incident wave 307 by its
impedance or impedance mismatch between passive network 303 and
antenna 302, such that reflected wave 309 can be represented by
A.sub.1e.sup.j.phi.1, where A.sub.1 is the relative amplitude of
backscatter and .phi.1 is the relative phase.
[0051] In the special case of A.sub.i=1, the corresponding i'th
passive network is lossless. If A.sub.i is essentially equal to 1,
then the corresponding passive network is substantially lossless.
"Substantially lossless" can include amplitude coefficients within
1%, 5%, 10%, or greater of 1.
[0052] Other embodiments can have the `reflected` or modified wave
travel through a separate path than that from which it came. The
separate path can lead to another antenna, such that the receive
antenna is separate from the transmit/emit antenna.
[0053] While the wireless signal is received into antenna 302,
switch 306 can be switched between passive networks or circuits
303, 304, . . . , 305 such that reflected waves or output signals
from each network temporally combine to form an output signal along
the common (i.e. left terminal in switch 306), and an output signal
from antenna is thus modulated in predetermined amplitude and
predetermined phase. The predetermined amplitude/phase modulation
corresponds to an identifier, serial number, or other device
signature of RFID tag 300. The output signal is emitted from RFID
tag 300 through antenna 302.
[0054] Switch 306 can be made from transistors, PIN diodes,
micromechanical or other switches as known in the art. Solid state
switches can be made from silicon, gallium arsenide, or other
semiconductor materials.
[0055] FIG. 4 shows a microstrip patch 400, which is a minimum
scatter antenna. Patch 403 is a rectangular piece of conductive
material. Circular, triangular, other simple shapes, and more
complex, arbitrary patterns can also be used successfully for a
patch element. Ground 402 lies in a parallel plane to patch 403,
separated by dielectric 401. Dielectric 401 can be made from
plastic; low loss dielectric material is preferred. In the
exemplary embodiment, patch 403 is the main element in 400 and
ground plane 402 may be considered the parasitic element.
[0056] FIG. 5 shows an exemplary scheme for generating a Binary
Phase Shift Keyed (BPSK) scattered signal. Patch element 502 is
similar to patch element 403 in FIG. 4. Patch element 502 has
radiating edges 501a and 501b and non-radiating edge 505. One
terminal of on/off switch 507 (i.e. single pole, single throw
(SPST) switch) is connected to a point on non-radiating edge 505.
The other end of switch 507 is connected to the ground plane (see
FIG. 4) through a via hole 504.
[0057] While an electromagnetic wave is received into patch element
502, on/off switch 507 is operated to modulate the output signal.
The phase of the radar cross section of patch antenna 502, rather
than its amplitude, is changed by the switching process. This
predetermined modulation corresponds to the identifier of the RFID
tag.
[0058] FIG. 6 shows an exemplary scheme for generating a Quaternary
Phase Shift Keyed (QPSK) scattered signal. The common terminal of
four-position switch 606 (i.e. single pole, quadruple throw switch)
is connected to non-radiating edge 608 of the patch. Circuits 601a
and 601b are via holes to the ground plane connecting two different
positions of 606. Circuit 601a generates a short circuit to ground,
whereas circuit 601b is connected through shorted transmission line
603, generating an effective inductance. Transmission line 603 can
be a simple transmission line, an inductor component, or other
inductor. Terminal or position 609 of switch 606 generates an open
circuit. Capacitive stub 604 is connected to the fourth position of
606. The capacitive stub can be a triangle shaped metallic pattern
or other metallic patterns on a dielectric. Lumped capacitors can
also be used. Therefore, four phase shifts spaced at 90.degree.
apart can be generated by this scheme.
[0059] FIG. 7 illustrates how a clutter signal phasor 703 (at the
reader) affect a signal from a tag. A `low state` from the RFID tag
is represented by the phasor 701 and a `high state` by phasor 702.
The resultant signals, as received by the reader are represented by
phasors 705 and 706, in the low and high states of the tag
respectively.
[0060] The following signals can be defined at the reader's
receiver as follows (phase shift keyed signal):
se.sup.j0=signal from the tag alone--low state (e.g. phasor 701);
(Eqn. 1)
se.sup.j.psi.=signal from the tag alone--high state (e.g. phasor
702); and (Eqn. 2)
me.sup.j.mu.=signal from impairments (reflection/scattering from
non-tag objects and transmitter leakage) alone (e.g. phasor 703),
(Eqn. 3)
where .psi. is the phase shift between the low and high states of
the tag signal.
[0061] FIG. 8 is a plot generated through mathematical modeling
showing how the phase angle .psi. between phasor 705 and phasor 706
changes as a function of .mu.. The phase shift between low and high
states .psi. is used as a parameter (90.degree. and 180.degree.).
m/s=5 was used in this plot.
[0062] FIG. 9 shows algorithm 900 to distinguish between low and
high states from the tag in presence of heavy clutter. After
beginning at step 902, the phase shift states are set in step 904
at the tag to 0.degree. and 180.degree.. In step 906, phase angles
are measured at the reader for high and low levels emitted by the
tag. In step 908, the algorithm determines whether the phase
difference between the measured angles is discernible. If it is
determined in step 908 that the phase difference is not
discernible, then the phase shift states are set at the tag to
0.degree. and 90.degree. in step 910. In step 912, the algorithm
determines again whether the phase difference between the measured
angles is discernible. If it is determined in step 912 that the
phase difference is still not discernible, then the data is
presumed lost in step 914 and the algorithm ends at step 916. If
the phase difference is discernible either in step 908 or step 912,
the algorithm then determines in step 918 whether the data is noisy
(e.g. includes clutter). If it is determined that the data is
noisy, then a correction is performed in step 920. The data is then
decoded in step 922 and the algorithm ends at step 924.
[0063] FIG. 10 depicts a correction algorithm 1000 whereby an
estimate of the clutter phasor is estimated and subtracted from the
received signals at a reader to determine the low and high states
from an RFID tag. After beginning at step 1002, step 1004 computes
an estimate of low and high level signals. Step 1006 computes an
estimate of the magnitude and phase of the impairment signal (e.g.
E[me.sup.j.mu.]). In step 1008, the estimate is subtracted from the
signal received back from the tag. The algorithm then moves on to
the next step 1010 of processing.
[0064] The estimates of signal at the reader in low and high states
of the tag are:
E[L]=E[se.sup.j0+me.sup.j.mu.] (e.g. phasor 705--low level signal);
and (Eqn. 4a)
E[H]=E[se.sup.j.psi.+me.sup.j.mu.] (e.g. phase 706--high level
signal). (Eqn. 4b)
Then, E[s](1-e.sup.j.psi.)=E[L]-E[H], such that:
E[s]=|(E[L]-E[H])/(1-e.sup.j.psi.)| (Eqn. 5a)
and
E[me.sup.j.mu.]=1/2[E[L]+E[H]-E[s](1+e.sup.j.psi.)] (Eqn. 5b)
Therefore, it is possible to calculate E[me.sup.j.mu.] from above
equations. Afterward, the corrected signal is obtained by
subtracting E[me.sup.j.mu.] from the received signal.
[0065] FIG. 11 illustrates system 1100 in which reader 1102 reads a
wireless signal from RFID tag 1120. Reader 1102 includes display
1104, memory 1106, microprocessor 1108, and data bus 1110. Reader
1102 also includes reader radio frequency (RF) antenna 1114
connected to the other components through interface 1112. Reader
transmits wireless interrogation signal 1116, which is received by
RFID tag 1120. In particular, RFID tag antenna 1124 receives the
interrogation signal and, through active switching of switch 1122
connecting to two networks (e.g. open circuit and ground circuit),
modulates the scattered wireless signal 1118. Reader RF antenna
1114 receives wireless signal 1118. Interface 1112 filters,
amplifies, and coherently demodulates the signal. Based on the
complex demodulated symbols, the microprocessor 1108 estimates an
amplitude and phase of the signal in two or more distinct states
(e.g. low and high states of the RFID tag). Microprocessor 1108
then estimates an amplitude and phase of complex clutter received
in signal 1118. Microprocessor 1108 then uses the estimate of the
amplitude and phase of the complex clutter in the received signal
to remove or otherwise subtract complex clutter from the received
signal. In this way, microprocessor can determine the corresponding
device ID of RFID tag 1120 among other tags and clutter.
[0066] In the foregoing specification, the invention is described
with reference to specific embodiments thereof, but those skilled
in the art will recognize that the invention is not limited
thereto. Various features and aspects of the above-described
invention may be used individually or jointly. Further, the
invention can be utilized in any number of environments and
applications beyond those described herein without departing from
the broader spirit and scope of the specification. The
specification and drawings are, accordingly, to be regarded as
illustrative rather than restrictive.
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